1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, printer, or fax machine and, more particularly, to a multibeam image forming apparatus that exposes a photoconductor with a plurality of laser beams to form an image.
2. Description of the Related Art
As an example of an apparatus that exposes a photoconductor with a plurality of laser beams to form an image, a multibeam image forming apparatus using a polygon mirror is known. The image forming apparatus generates a plurality of laser beams at a time using the polygon mirror to exposure-scan a photoconductor drum. In forming a color image, the image forming apparatus exposes a plurality of photoconductors spaced apart from one another with a given interval with laser beams and, after development, transfers the color image onto a paper fed by a feeding belt.
In the case where a plurality of laser beams are used to form a color image as described above, exposure scanning operations for respective colors must be synchronized. If the operations are out of synchronization, image distortion, color misalignment, or line misalignment occurs. To cope with this problem, a light sensor (also referred to as BD sensor) that detects a laser beam is disposed at a start position of scanning operation of the polygon mirror for the laser beam to obtain a synchronization signal (Beam Detect signal, also referred to as BD signal) serving as a criterion for starting the scanning operation and writing start timing of laser beam is determined for each color based on the synchronization signal.
Actually, however, displacement of optical elements constituting an optical system, a change in rotation balance resulting from micro vibration of rotator components due to increase in temperature of the polygon mirror, and the like cause the timing detected by the BD sensor to fluctuate, making it difficult to perform an accurate detection. Considering BD signal obtained from a first laser beam as a reference, if the phase of BD signal obtained from a second laser beam is shifted back and forth, the writing start timing of laser beams corresponding to respective colors becomes out of synchronization, causing color misalignment. In some cases, starting positions of scanning operation for photoconductors corresponding to respective colors are shifted by one line each.
An image forming apparatus using a plurality of laser beams that prevents misalignment in the start timing of image exposure is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2004-98449.
The image forming apparatus disclosed in the publication detects reference points of line scans of a plurality of laser beams with a light sensor, generates synchronization signals corresponding to respective laser beams, and determines the start timing of image exposure based on the synchronization signals. After that, the image forming apparatus determines one reference synchronization signal from among the synchronization signals and uses a delay means to delay another synchronization signal whose generation timing is close to the reference synchronization signal to make a timing difference between synchronization signals large, thereby determining the start timing of image exposure.
In this example, it is necessary to determine a synchronization signal that is different from the reference synchronization signal and whose generation timing is close to the reference synchronization signal. Further, a delay means is used to make a timing difference large, causing color misalignment in some cases.
Further, Jpn. Pat. Appln. Laid-Open Publication No. 2004-98299 discloses an image forming apparatus using a plurality of laser beams that determines BD signal obtained from a first laser beam as a reference at the time of generation of a horizontal synchronization signal (BD signal), previously measures a phase difference between the reference BD signal and BD signal obtained from another laser beam, and determines the write timing of an image according to the measured phase difference.
In this example, it is necessary to previously measure the phase difference. Further, the phase difference is varied in some cases due to displacement of optical elements constituting an optical system, temperature change, or mechanical micro vibration of a polygon mirror. Thus, the measured phase difference does not always correspond to the estimated value, causing color misalignment in some cases.